1. Field of the Invention
The present invention relates to a superconducting coil, a superconducting magnet, and a method of operating the superconducting magnet and particularly to quench protection for a superconducting coil in a superconducting magnet operated in a persistent mode.
2. Description of the Related Art
Because a superconducting magnet used in an MRI (Magnetic Resonance Imaging) apparatus, an NMR (Nuclear Magnetic Resonance) or the like generally require a high intensity magnetic field, a magnetic energy (LI2/2, where L is an inductance and I is driving current) stored in the coil of a superconducting magnet becomes large.
Accordingly, the superconducting coil requires a protection technology for preventing the superconducting coil from burning due to local Joule heating when quench (transition from a superconducting state to a normal state) occurs in the superconducting coil.
As the quench protection, a method of consuming energy in a protection resistor connected in parallel to the superconducting coil is known.
In this method, because when the superconducting coil is driven by a current supplied from a power source in a driven mode, it is possible to conduct the current through the protection resistor forcibly by cutting off the power source, this method is efficient as quench protection.
However, in a persistent mode continuing the current flow through a closed circuit including the superconducting coil and a persistent current switch, the circuit cannot be forcibly opened. This method only divides the current into the protection resistor by the resistance generated by quench (see FIG. 7 of JP 61-74308).
Therefore, it is important to increase the resistance generated when the quench, i.e., when the quench occurs, it is important to rapidly spread the quench area (normal conducting region) over the whole of the coil.
In addition, various methods of quench protection have been developed.
JP 61-74308 discloses a protection method of connecting a diode in parallel to the superconducting coil instead of the protection resistor to suppress a current flowing through a protection circuit when excitation is cut off using the switching voltage.
In JP 2007-234689 discloses a method of protecting the superconducting coil in a case where a plurality of superconducting coils are connected in series. In the method, a protecting circuit is configured with a diode and a heater. When quench occurs, a current flows through the heater by a potential difference due to the quench to induce quench in all superconducting coils.
Quench protection technologies for superconducting magnets (superconducting coils) using low temperature superconducting wires such as a niobium-titanium alloy (NbTi) are known. On the other hand, the quench protection becomes more difficult in superconducting magnets (superconducting coils) using a high temperature superconducting wire including, for example, a magnesium diboride (MgB2) than the case using the low temperature superconducting wire.
On the other hand, because the high temperature superconductor has a higher critical temperature, it is possible to make a difference between an operation temperature and the critical temperature larger. In addition, because the higher the temperature of the high temperature superconductor, the larger specific heat the high temperature superconductor has, the high temperature superconductor has a merit in that it is not easy for quench to occur.
However, when the quench occurs in the superconducting coil due to a power fail or a trouble in a refrigerator, this advantage turned to be a demerit.
More specifically, the fact that the quench hardly occurs because there is a large difference between the operation temperature and the critical temperature and the specific heat is large corresponds to a result that the quench hardly spread because there is the large difference between the operation temperature and the critical temperature and the specific heat is large when the quench locally occurs. When an area where the quench occurs is narrow, a temperature of the area rapidly increases because the energy in the superconducting coil is locally consumed. Accordingly, the temperature may instantaneously exceed a burning temperature.
In addition, as described in JP 2007-234689, even if the method of accelerating spreading of quench with the heater is used, there is a possibility of burning before the quench is sufficiently spread in consideration of time necessary for quench detection, heating and temperature increase with the heater. Particularly, the high temperature superconductor has a larger difference in temperature between the operation temperature and the critical temperature and has a larger specific heat than the low temperature superconductor, so that it takes for a long period to increase the temperature of the superconducting coil from the operation temperature above the critical temperature. During this, risk to burning of the superconducting coil is high.